a. Field of the Invention
The present invention relates to a ultra-wide angle photographic lens system to be used with single-lens reflex cameras and, more particularly, to a retrofocus-type photographic lens system of F/3.5 attaining a wide field angle of 100.degree..
B. Description of the Prior Art
For ulra-wide angle photographic lens systems for single-lens reflex cameras, it is necessary to make the back focal lens long in order to avoid "mirror-up" and, when the focal length of the lens system as a whole is represented by reference symbol f, the back focal length has to be made 2f or more. Besides, in this kind of lens systems, there is a tendency that the front lens diameter becomes large. When, however, the front lens diameter is large, an inconvenience occurs because it is necessary to use filters of large diameters exclusively for that lens system. To eliminate the above-mentioned inconvenience, it is desirable that the front lens diameter is small. Besides, in retrofocus-type lens systems in which a front lens group is arranged as a negative lens group and a rear lens group is arranged as a positive lens group, the lens system is asymmetrical. Consequently, when the field angle becomes wider, there is a tendency that distortion of higher degree and lateral chromatic aberration are caused.